1. Field of the Invention
This invention is generally related to integrated circuit packaging and, more particularly, to packaging of high speed devices having sensitive structures, such as air bridge structures.
2. Description of the Related Art
In the packaging of very large scale integrated circuit devices, much space is taken up by connections between integrated circuit (IC) chips and adjacent chips and/or other devices. In a present practice designed to reduce packaging sizes, a polymer film overlay covers a plurality of integrated circuit chips adjacent to one another on an underlying substrate. The polymer film provides an insulated layer upon which is formed a metallization pattern for interconnection of individual circuit chips. A significant advantage of these packaging configurations is the ability to remove one or more of these interconnection layers so as to provide a multitude of arrangement and testing capabilities.
Methods for carrying out a high density interconnection (HDI) process using overlays are described in Eichelberger et al., U.S. Pat. No. 4,783,695, issued Nov. 8, 1988, and in Eichelberger et al., U.S. Pat. No. 4,933,042, issued Jun. 12, 1990, both of which are commonly assigned and hereby incorporated by reference. Additionally, it is desirable to provide via openings or apertures in the polymer dielectric layer so as to be able to provide electrical connection between various parts of a chip or between several chips. Commonly assigned Eichelberger et al., U.S. Pat. No. 4,894,115, issued Jan. 16, 1990, which is hereby incorporated herein by reference, describes embodiments for providing such apertures. Furthermore, methods for gaining access to and replacing a defective integrated circuit chip are described in commonly assigned Eichelberger et al., U.S. Pat. No. 4,878,991, issued Nov. 7, 1989, which is hereby incorporated by reference.
For the fabrication of certain multi-chip module (MCM) devices, high performance is attained by using high speed devices made of materials such as GaAs (gallium arsenide), for example, some of which have chip wiring including air bridge structures. The thermal stability requirements of certain GaAs chips restrict their processing range to temperatures under 260.degree. C. Therefore, for processing modules with these chips, a lower melt viscosity adhesive system was developed to provide a procedure compatible with processing temperatures below 260 .degree. C., and is disclosed in commonly assigned Lupinski et al., "Plasticized Polyetherimide Adhesive Composition and Usage", U.S. Pat. No. 5,300,812, which is herein incorporated by reference.
Air bridge structures provide improved signal propagation and reduced capacitive coupling at high frequencies (in the gigahertz range) over conventional chip wiring. To maintain the performance advantage of having air as the dielectric medium, the MCM fabrication process must preserve the air bridge structures. For example, allowing adhesive underneath air bridges with a dielectric constant of about 3 instead of with a dielectric constant of 1 will cause additional capacitive loading and reduction of signal magnitude at high frequencies.
One high density interconnection (HDI) technique for chips with air bridge structures is to fabricate a multi-layer interconnection structure by laminating polymer layers and forming electrical conductor patterns and to then selectively remove the polymer layers from the air bridge regions of the chip using a large area laser ablation process. A plasma treatment is used to clear any remaining residue. This technique is suitable for large air bridge structures with good structural integrity. Certain high performance chips with fragile air bridge structures, however, can be crushed or deformed when an initial layer of polymer is laminated.
In another technique, a low temperature, low melt viscosity adhesive is used between the overlay polymer and chip during lamination to prevent thermal degradation of sensitive chips. The adhesive flows under air bridge structures, however, and cannot be completely removed after large area ablation.